The invention relates to a process for investigating the activity of a cathodic protection unit installed on a metal structure that is located under water or in groundwater. The invention further relates to an apparatus suitable for carrying out that process.
Cathodic protection is a method for combating corrosion of metals that are exposed to the action of water. A great variety of constructions, such as pipelines, offshore drilling and production platforms, piers, sluice-gates and ships, are protected in this way.
Metal corrosion in an electrolyte at atmospheric temperature is an electrochemical process. At the surface of the corroding metal anodic and cathodic sites develop, whose distribution over the metal surface is dependent on the condition of this surface and is affected by inclusions, oxide layers, etc. The metal of the anodic sites will pass into solution and at the cathodic sites reduction of the oxygen present in the electrolyte place. In the electrolyte an electric current begins to flow from the anodic to the cathodic sites.
The principle of cathodic protection is the prevention of the potential-dependent anodic solution reaction of the metal. This aim is achieved when no current flows through the electrolyte from the anodic to the cathodic sites. A decrease of the potential of the metal from the corrosion potential to the protection potential is obtained by passing the current required for the cathodic reduction reaction from an external source through the electrolyte to the metal, so that this current is no longer supplied by the passing into solution of the metal. Two methods are used for this purpose. In the case of protection by impressed current -- active cathodic protection -- the object to be protected is coupled to a direct-current source which causes a current to flow via an anode system to the object. The process using sacrificial anodes -- passive cathodic protection -- is based on the formation of a galvanic cell when the object to be protected is coupled to a less noble metal that acts as the anode and passes into solution, so that at the wall of the object only oxygen reduction occurs.
Passive cathodic protection of steel in seawater may be achieved with magnesium, often alloyed with 6% Al and 3% Zn. Also very suitable anode material is aluminum, often alloyed with 4% Zn and 0.02% In. A sacrificial anode for prolonged use may have a weight of 500 kg. Soft iron is a very suitable anode material for the protection of alloys containing copper.
Whichever the system used, protection is only obtained if a certain current flows through the electrolyte from the anode to the object to be protected. However, in most cases it is not the current density that is chosen as criterion, but the potential relative to a reference electrode, which can be measured more easily. For iron in aerated seawater the protection potential lies somewhere near -780 mV. This potential is determined by connecting a voltmeter with a high input impedance to the protected construction and to a reference electrode placed close to it. Reference electrodes may be a calomel, a copper sulphate, a silver chloride, or a hydrogen electrode, but also, for instance, zinc. With large structures, for instance a submarine pipeline, direct measurement of this potential is very cumbersome or even impracticable. Regular checking of the proper functioning of a cathodic protection unit is, however, of great importance in view of the reliability of the structure. The invention indicates how a reliable continuous or at least regular inspection of a cathodic protection unit can be carried out.